Numerical controls (NC) are used primarily for controlling machine tools, and may be subdivided essentially into two functional units, a main computer and at least one controller unit.
The main computer makes available the user interfaces such as keyboard and monitor necessary for operating the NC, and is used for creating, storing and executing programs. Located in the controller units are control loops which are used for driving converter modules, that in turn drive motors. The controller units also include digital and/or analog interfaces for acquiring actual values needed continuously during a program execution for controlling the control loops. The actual values to be acquired may, for example, be position values, speed values, acceleration values or also current values. Similar to the main computer, the controller units are also microprocessor-controlled.
There may be the desire to spatially separate the main computer and the controller units. Thus, it may be provided to combine the main computer together with the keyboard and the monitor in one housing, in order to create a user interface permitting optimal placement for the user from the standpoint of ergonomics. It may be desirable to arrange the controller units close to the converters, to ensure optimal signal quality of the pulse-width-modulated control signals.
Serial interfaces present themselves for the data transmission between spatially separated units, since cables having only a few wires may be used, which may be very inexpensive and easy to manipulate compared to cables for data transmission via parallel interfaces.
The transmission of safety-related information may be regarded as problematic in such a system architecture, since the microprocessor-controlled units determine what data is transmitted via the serial interface. Therefore, in the event a microprocessor-controlled unit malfunctions, caused, for example, by a program crash or by external influences (e.g., fluctuations in the operating voltage, overheating of modules), it may happen that safety-related information will not be transmitted. Particularly in the case of an emergency shutdown due to safety, this can have dangerous consequences ranging from material damage to the machine tool, right up to endangering life and limb of the operating personnel.
U.S. Pat. No. 5,274,311 describes a modular motor-control system, which includes a motor-control module, an input/output module and a processor module which communicate with each other via a differential two-wire interface. For reliable transmission of a shut-off signal, a separate line is provided that is parallel to the two-wire interface to all modules. Reliable shutdown may thereby be ensured, even if the data transmission via the two-wire interface fails.
Considered to be disadvantages of this design approach are first of all, that an additional line is needed, and secondly, that only a dedicated signal can be transmitted via the one additional line.